Electromagnetic Acoustic Transducers (EMATs) are advanced ultrasonic transducers that generate and detect acoustic waves in an electrically conducting material without making physical contact with the material unlike its counterpart, the piezoelectric transducers (PZT). The conventional EMAT consists of copper coil that generates the dynamic field when excited with a sinusoidal current, a permanent or electromagnet that provides the bias field and the conducting material specimen. The complex interaction between the bias field and the Eddy current induced within the skin depth of the conducting material by the dynamic field gives rise to the acoustic wave that then propagates within the surface of the material. Within the research a finite element EMAT model was developed using commercial software Comsol Multiphysics, to study and compare the Eddy current density and Lorentz force density generated by three EMAT configurations: The Meander-line, Spiral and Key Type EMAT configuration respectively. It was observed that apart from the ease of fabrication and simplicity of connectivity when stacked in layers, the Key Type coil EMAT showed a high tendency to generate higher amplitude of Eddy current and Lorentz force test materials especially when stacked in layers. Also, the effect of varying some key EMAT parameters was investigated to determine the optimal performance of Key Type EMAT configuration on CS70 pipe steel plate. The research further developed a coupled finite element model using the same software, Comsol Multiphysics to account for the generation, propagation and detection of acoustic wave by the Key Type EMAT configuration on CS70 grade of pipe steel. The model can solve the magnetostatic, electrodynamic and elastic equations that give rise to acoustic wave generation, propagation and detection on the test material. The developed coupled finite element model was validated both analytically and experimentally to establish the validity of the finite element model. The analytical and experimental results obtained were consistent with the numerical result with an average discrepancy less than 9 % percent. Finally, the research developed a novel modelling strategy to decouple and quantify the various transduction forces in operation when normally-biased EMAT and magnetostrictive EMAT configurations are used on various grades of pipe steel materials. The strategy established the value of the critical excitation current beyond which acoustic wave is generated solely by the dynamic Lorentz force mechanism. The critical excitation currents when Magnetostrictive EMAT configurations are used to generate acoustic wave was found to be; 268A, 274A, 279A, 290A and 305A for CS70, L80SS, L80A, TN80Cr3 and J55 respectively. While for Normally-Biased EMAT configurations, the critical excitation current was found to be 190A, 205A, 240A, 160A and 200A respectively. This work also compared the critical excitation current of the two EMAT configurations studied and established that normally-biased EMATs are more efficient in the generation of acoustic waves than their magnetostrictive counterpart due to their lower value of critical excitation current.

Electromagnetic acoustic transducers (EMATs) are increasingly used in industries due to their attractive features of being non-contact, cost-effective and the fact that a variety of wave modes can be generated, etc. There are two major EMATs coupling mechanisms: the Lorentz force mechanism for conductive materials and the magnetostriction mechanism for ferromagnetic materials; EMATs operated on Lorentz force mechanism are the focus of this study. This work aims to investigate novel efficient modelling techniques for EMATs, in order to gain further knowledge and understanding of EMATs wave pattern, how design parameters affect its wave pattern and based on above propose and optimise novel sensor structures. In this study, two novel modelling methods were proposed: one is the method combining the analytical method for EM simulation and the finite-difference time-domain (FDTD) method for US simulation for studying the Rayleigh waves' properties on the vertical plane of the material; the other one is the method utilizing a wholly analytical model to explore the directivity of surface waves. Both simulations models have been validated experimentally. The wholly analytical model generates the radiation pattern of surface waves, which lays a solid foundation for the optimum design of such sensors. The beam directivity of surface waves was investigated experimentally, and results showed the length of wires has a significant effect on the beam directivity of Rayleigh waves. A novel configuration of EMATs, variable-length meander-line-coil (VLMLC), was proposed and designed. The beam directivity of surface waves generated by such novel EMATs were analytically investigated. Experiments were conducted to validate such novel EMATs models, and results indicated that such EMATs are capable of supressing side lobes, and therefore resulting in a more concentrated surface waves in the desired direction. Further, another two novel configuration of EMATs, the four-directional meander-line-coil (FDMLC) and the six-directional meander-line-coil (SDMLC), were proposed and designed; results showed these EMATs are capable of generating Rayleigh waves in multiple directions and at the same time suppressing side lobes.

[pt] Ondas ultrassônicas podem ser usadas para ensaios não destrutivos. Elas são geradas e adquiridas por transdutores. Transdutores acústicos eletromagnéticos (EMATs) possuem algumas vantagens em relação a transdutores tradicionais piezoelétricos, principalmente a capacidade de gerar ondas ultrassônicas sem necessidade de contato físico com o meio em teste. No entanto, sua principal desvantagem é a menor eficiência, que resulta em uma relação sinal-ruído mais baixa. Técnicas de casamento de impedância podem ser utilizadas para combater isso. A rede de casamento de impedância do tipo circuito L é comumente utilizada para garantir a transferência máxima de potência da eletrônica de excitação para o EMAT. Existem diversas variáveis que podem afetar a impedância de um EMAT além do próprio transdutor, como as propriedades e distância do material, a temperatura e a frequência de excitação. Portanto, para garantir a transferência máxima de potência, o circuito de casamento de impedância precisa ser reconfigurado e ter seus valores ajustados sempre que um dos fatores mencionados acima sofrer alteração. O processo de ajuste manual desta rede é trabalhoso e demorado, portanto, sua automação pode trazer grandes benefícios para o uso de transdutores EMAT. Esta dissertação propõe um circuito simplificado, com um único elemento, para casamento de impedância de EMATs. Os valores teóricos ideais para circuitos mono-elemento foram obtidos. Simulações confirmaram sua viabilidade em aumentar a eficiência do EMAT. Circuitos de casamento manual e automáticos foram projetados e construídas. Configurações experimentais foram elaboradas e postas em prática. Experimentos com dois transdutores EMATs distintos foram conduzidos utilizando várias frequências. O sistema automático foi capaz de determinar a melhor configuração para o circuito mono-elemento de casamento de impedância e forneceu um ganho de até 5,6 dB, similar à solução manual. A configuração automática foi mais de duas vezes mais rápida do que a manual. / [en] Ultrasonic waves can be used for nondestructive testing. They are generated and acquired by transducers. Electromagnetic acoustic transducers (EMATs) have some advantages over traditional piezoelectric transducers. Mainly, the ability to generate ultrasonic waves without requiring physical contact with the medium under test. Nevertheless, they present a main drawback of less efficiency, which leads to a lower signal-to-noise ratio. To overcome this, impedance matching techniques can be used. The L-network impedance matching network is often used in order to ensure maximum power transfer to the EMAT from the excitation electronics. There is a wide range of variables that can affect an EMAT s impedance besides the transducer itself, namely, the properties and distance to the specimen material, the temperature, and the excitation frequency. Therefore, to ensure optimal power transfer, the Lnetwork s configuration needs to be tuned whenever one of the factors that affect impedance changes. The process of manually adjusting the impedance matching network is a laborious and time-consuming task, therefore, its automation can be of great benefit to the use of EMAT transducers. In this work, a simplified one-element automatic matching network is proposed. The theoretical optimal values for the one-element matching networks are derived. Simulations confirmed their effectiveness to increase EMAT efficiency. Manual and automatic networks were designed and built. Experiments were performed with two dierent EMATs at several frequencies. The automatic system was able to determine the best conguration for the one-element matching network and provided up to 5.6 dB gain, similar to the manual solution. The automatic setup was more than two-fold faster than the manual one.

[pt] Ondas ultrassônicas do tipo SH (do inglês, shear horizontal), ou de cisalhamento horizontal são úteis no campo de ensaios não-destrutivos. Essas podem ser geradas, em meios metálicos, por transdutores acústicos eletromagnéticos (EMAT) que utilizam um arranjo de ímãs permanentes periódicos (PPM), chamados de EMATs PPM. Tais dispositivos em sua forma convencional, entretanto, geram ondas ultrassônicas bidirecionalmente, ou seja, para frente e para trás. Essa característica é geralmente indesejada. Recentemente, foi proposto um EMAT tipo PPM duplo com separação lateral que gera ondas nominalmente em uma única direção. Entretanto, seu diagrama de radiação revela lóbulos traseiros laterais. Esta dissertação tem como objetivo aprimorar o supracitado projeto através de seus parâmetros de construção, mais especificamente, o número de fileiras de ímãs permanentes do arranjo PPM e a separação lateral entre as mesmas. Foi desenvolvido um modelo analítico, baseado em linhas de fontes lineares, que considera a projeção de cada espira como uma fonte harmônica independente, para cálculo do campo ultrassônico gerado por EMATs PPM, tanto os do tipo PPM duplo, quanto o convencional. Com isso, foi possível obter o digrama de radiação teórico dos dispositivos, de forma a analisar como os seus parâmetros de construção interferem em sua unidirecionalidade. Utilizou-se três tecnologias distintas de bobinas, a saber, bobinas fabricadas manualmente, bobinas fabricadas a partir de placas de circuito impresso (PCB) flexível de poliamida e PCB flexível de poliéster. Os arranjos PPM duplos foram colocados sobre moldes impressos em impressora 3D. Realizou-se testes experimentais em uma placa de alumínio 1.5 mm de espessura, com EMAT PPM comercial utilizado como receptor, em diversos ângulos, com o objetivo de calcular o diagrama de radiação experimental do EMAT PPM duplo. Ao todo foram montadas 44 unidades quando combinadas todas as tecnologias de fabricação de bobina, com separação lateral de 1 mm a 4 mm e 1 a 4 de fileiras de ímãs. Resultados experimentais e teóricos mostraram boa concordância. Foi observado que tanto aumentar o número de fileiras de ímãs, quanto diminuir a sua separação lateral reduzem a intensidade dos lóbulos traseiros laterais. Resultados experimentais mostraram que, com a configuração de 4 fileiras de ímãs, separados lateralmente por 1 mm e bobinas feitas a partir de PCB de poliamida, a intensidade dos lóbulos traseiros laterais foi reduzida em 8.6 dB, em relação à configuração original do PPM EMAT duplo . Sobre as tecnologias de bobina, pode-se concluir que o uso de PCBs flexíveis tornou possível obter menores separações laterais entre as fileiras de ímãs e simplificar o processo de construção do EMAT PPM duplo, com destaque para a bobina feita com PCB flexível de poliamida, que apresentou baixo erro médio entre medidas experimentais e simuladas, apresentando também o mais fácil processo de fabricação entre os três tipos de bobinas avaliados. / [en] Shear horizontal (SH) ultrasonic waves are useful in the non-destructive evaluation field. They can be generated, in metallic media, by electromagnetic acoustic transducers (EMAT) that use an array of periodic permanent magnets (PPM). PPM EMATs, in their conventional form, generate forward and backward traveling SH waves. This feature is generally undesired. Recently, a side-shifted dual-PPM EMAT has been proposed. This device generates waves predominantly in a single direction. However, its radiation pattern reveals backward side lobes. This thesis aims to improve the aforementioned design through its construction parameters, more specifically, the number of rows of magnets in the PPM array and the lateral separation between them. An analytical model, based on the line source method, which considers the wires projections as independent wave sources, was developed to calculate the ultrasonic field generated by PPM EMATs, either for the conventional one or the dual-PPM EMAT. Exploiting the developed model, it was possible to compute the theoretical radiation pattern of the device in order to better analyse how its construction parameters affect its unidirectionality. Three different coil technologies were used, namely hand-wound coils and coils made with flexible printed circuit boards (PCBs), either of polyamide or polyester material. The magnet arrays were mounted on 3D-printed molds to ensure their positioning. Experimental tests were carried out where the fabricated device was placed onto a 1.5 mm-thick aluminum plate, and a commercial PPM EMAT was used as a receiver, at different reception angles, in order to calculate the radiation pattern of the dual-PPM EMAT. Overall, 44 units, combining all coil technologies, were manufactured with lateral separation from 1 mm to 4 mm and 1 to 4 rows of magnets per PPM. Experiments and theoretical results presented good agreement. It was observed that either increasing the number of rows of magnets or decreasing their lateral separation reduced the intensity of the backward side lobes. When using the configuration of 4 rows of magnets and 1 mm side shift, fabricated with polyamide PCBs, results revealed that the intensity of the backward side lobes was reduced by 8.6 dB, when compared with the original configuration. Regarding the coil technologies, it can be concluded that the use of flexible PCBs allowed one to obtain shorter lateral separations between rows of magnets and simplified the fabrication process of the dual-EMAT. Mainly, the poliamide-based flexible PCB coil presented low average error between experimental and simulated measurements and the easiest fabrication process among all three coil technologies tested.